The development of high power lasers has put a premium on the development of window materials having absorption coefficients of the order of 10.sup..sup.-3 cm.sup..sup.-1 or lower for wavelengths corresponding to the lasers of interest. Heretofore measurements have been made by measuring the optical properties by conventional techniques at the wavelength of interest (i.e., transmittance, reflectance, and/or emittance). These techniques generally require extreme sensitivity and are limited to absorption coefficient-length products (.alpha.d) of 10.sup..sup.-2. Usually it is necessary to attach a thermocouple to the specimen in order to measure the rate of change of the change in temperature of the specimen. The sensitivity of thermocouples seriously limits the measurements of .alpha.d to the order of 10.sup..sup.-3 and have additional disadvantages of responding to incident radiation and are difficult to attach to the specimen under test.